The present invention relates to a method of monitoring a reservoir, in particular a reservoir made of composite material, preferably intended for storing gas at high pressure, the reservoir being provided with at least one acoustic sensor, the method being characterized in that it comprises the following steps: filling the reservoir with a fluid, acquiring data relating to the amplitude and the energy of acoustic emissions resulting from the filling, with the aid of the sensor or sensors, determining a critical threshold value (Sc) on the basis of a linear combination between data relating to the amplitude and the energy, as a function of this critical threshold value (Sc), deducing therefrom a state of the reservoir. The invention also relates to an installation for implementing the aforesaid method.